Sulfur is typically an incidental impurity in copper alloys, and is typically present in an amount that does not exceed 0.005% by weight. In fact, it is disclosed in the published literature that sulfur in an amount greater than 0.01% by weight may negatively impact certain copper-based alloys, causing the copper alloy to become brittle.
However, it is known to add sulfur to copper alloys to impart improved machinability. More specifically, sulfur has been added to molten ferrous and non-ferrous alloys (including copper alloys) to promote free-machining characteristics. The sulfur acts singularly or in combination with other alloy constituents to produce particles that act as chip breakers during machining. For example, U.S. Pat. No. 5,137,685 discloses the addition of sulfur to copper alloys, in combination with bismuth, in order to improve machinability. However, this patent expressly places an upper limit for sulfur at about 2% by weight, and indicates a preference for an amount of sulfur that is from about 0.1% to about 1.0%.
Generally, the insolubility of free sulfur and of sulfur-rich inter-metallic compounds in the alloy matrix is understood to determine the effectiveness of sulfur addition for improving machinability. It is widely believed that the known benefit of adding sulfur to a copper alloy is limited to improved machinability, and that the known copper alloys incorporating sulfur, even those exhibiting improved machinability due to sulfur addition, do not exhibit enhanced corrosion resistance.
From a historic perspective, lead has also long been added to copper alloys to improve machining of the alloys. Lead adds lubrication during machining and acts to break up machine chips. However, lead is potentially toxic, and it is desirable to limit contact of potable water with lead under conditions that may lead to the lead leaching into the water.